Hydraulic fracturing is a widely used method for stimulating petroleum producing subterranean formations and is commonly performed by contacting the formation with a viscous fracturing fluid having particulated solids, widely known as propping agents or proppants, suspended therein, applying sufficient pressure to the fracturing fluid to open a fracture in the subterranean formation, and maintaining this pressure while injecting the fracturing fluid into the fracture at a sufficient rate to extend the fracture into the formation. When the pressure is reduced, the propping agent within the fracture prevents the complete closure of the fracture.
The properties that a fracturing fluid should possess are amongst others, low leakoff rate, the ability to carry a propping agent, low pumping friction loss, and easy removal from the formation. Low leakoff rate is the property that permits the fluid to physically open the fracture and one that controls its areal extent. The rate of leakoff to the formation is dependent upon the viscosity and the wall-building properties of the fluid. Viscosity and wall-building properties are controlled by the addition of appropriate additives to the fracturing fluid. The ability of the fluid to suspend the propping agent is controlled by additives. Essentially, this property of the fluid is dependent upon the viscosity and density of the fluid and upon its velocity. Friction reducing additives are added to fracturing fluids to reduce pumping loss due to friction by suppression of turbulence in the fluid. To achieve the maximum benefits from fracturing, the fracturing fluid must be removed from the formation. This is particularly true with very viscous fracturing fluids. Most of such viscous fluids have built-in breaker systems that reduce the viscous gels to low viscosity solutions upon exposure to the temperatures and pressures existing in the formations. When the viscosity is lowered, the fracturing fluid may be readily produced from the formation. The use of aqueous based fluids to formulate fracturing fluids is well known. Such fluids generally contain a water soluble polymer viscosifier. Sufficient polymer is used to suspend the propping agent, decrease the leakoff rate, and decrease the friction loss of the fracturing fluid. Supplemental additives are generally required to further decrease the leakoff rate, such as hydrocarbons or inert solids, such as silica flour.
Various water soluble polymers have been used, or proposed for use as viscosifiers for aqueous based fracturing fluids, such as polyacrylamides, partially hydrolyzed polyacrylamides, and various polysaccharide polymers such as guar gum and derivatives thereof, and cellulose derivatives. Of these, guar gum and guar gum derivatives are the most widely used viscosifiers. Guar gum is suitable for thickening both fresh and salt water, including saturated sodium chloride brines.
It is known to provide concentrated suspensions of borate-containing crosslinking agents for the preparation of crosslinked fracturing fluids including hydroxyl functional polymers. See for example U.S. Pat. Nos. 4,514,309; 5,082,579; 5,145,590; and 5,160,643. U.S. Pat. No. 4,619,776 discloses the use of sparingly soluble borates for the controlled crosslinking of hydroxyl functional polymer-containing hydraulic fracturing fluids. Concentrated suspensions of such borates in hydrocarbon base fluids have been utilized for the crosslinking of fracturing fluids containing guar gum or derivatives thereof, particularly hydroxypropyl guar, and have achieved commercial success. These concentrates contain an organophilic clay suspending agent to keep the borate crosslinking agent suspended therein, thus preventing settling thereof. Additionally, U.S. Pat. No. 6,936,575 discloses aqueous suspensions of sparingly soluble borates thickened with palygorskite clays (aluminum silicate-based natural clay products), but requires a saturated borate solution as a starting material in forming the suspensions.
In aqueous environments, hydroxyl groups positioned in the cis-form on adjacent carbon atoms of a polymer, or on carbon atoms in a 1,3-relationship react with borates to form five or six member ring complexes. At pH above about 8.0, these complexes form crosslinked complexes. The reaction is fully reversible with changes in pH. An aqueous solution of the hydroxy-functional polymer will become highly viscous in the presence of borate anions when the solution is made alkaline, and will liquefy again when the pH is lowered below about 8. The critical pH at which gelation occurs may be modified by the concentration of dissolved salts, which are known to change the pH at which a sufficient quantity of dissociated borate ions exists in solution to cause gelation. The addition of an alkali metal base such as sodium hydroxide enhances the effect of condensed borates such as borax by converting the borax to the dissociated metaborate.
Known polymers which contain a substantial content of reactive hydroxyl groups include, for example, guar gum, locust bean gum, dextrin, polyvinyl alcohol, and derivatives of these polymers as well as mixtures of two or more thereof.
Depending on the relative concentration of polymer, and borate anion, the crosslinking reaction may produce useful gels, or may lead to insolubilization, precipitation, or unstable, non-useful gels. The viscosity of the hydrated polymer solution increases with an increase in the concentration of borate anion until a maximum is obtained. Thereafter the viscosity decreases and the gel becomes unstable as evidenced by a lumpy, inhomogeneous appearance and syneresis. As the temperature of the solution increases, the concentration of borate required to maintain the maximum degree of crosslinking, and thus maximum viscosity increases.
Concentrated aqueous suspensions of the sparingly soluble, alkaline earth or alkali metal alkaline earth metal borates have also been commercially successful. Such aqueous concentrates contain a high concentration of the sparingly soluble borate. The concentrates are added in the field to an aqueous hydraulic fracturing fluid containing the polymer to be crosslinked. Typical problems with the concentrates include settling of the solids prior to the use thereof in a crosslinking reaction. The sparingly soluble boron-containing particulates present in the aqueous suspension tend to settle because the particulates are substantially denser than water.
Thus, there is a need in the industry to develop concentrated aqueous suspensions of sparingly soluble, alkaline earth or alkali metal alkaline earth metal borates that are shelf stable and not prone to settling.